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RustFS could not read a single MinIO-encrypted object. Two independent blockers, and backlog#1638 could only argue them statically because the fixtures the interop tests consume are generated, not checked in — so those tests had never once run. With the fixture lab working, both are now measured, fixed and covered.
The detection gate required `x-amz-server-side-encryption` to be present. MinIO never persists it: `crypto.S3.CreateMetadata` writes only the `X-Minio-Internal-*` family and the public header is synthesized onto the response by `DecryptObjectInfo`. Every MinIO object therefore fell out of the managed path and failed with "encrypted object metadata is incomplete". The scheme is now inferred from which sealed-key slot is present, which is self-consistent by construction: the slot decides both which header the unseal reads and which domain string the sealing key is derived under, so an inference that disagreed with the slot could not silently derive a wrong key. Inferring from the KMS key id would NOT be safe — MinIO writes `-S3-Kms-Key-Id` on SSE-S3 objects too, which the fixtures show and a mutation test pins.
Past the gate, the data key itself could not be unwrapped. Its wire format is `sealed_bytes || iv[16] || nonce[12]` — the randomness trails the ciphertext rather than leading it — with a per-ciphertext sealing key of `HMAC-SHA256(master, iv)` and the encryption context bound as associated data (`internal/kms/secret-key.go`). Note this is not the `{"aead":...}` JSON that backlog#1638's analysis described: current MinIO writes the raw layout and treats JSON only as a legacy encoding, normalizing it into the same byte order. Both are decoded here, in a decoder of their own — `LocalSseDekEnvelope`'s `deny_unknown_fields` is untouched, since loosening it to admit MinIO's shape would also admit malformed RustFS envelopes that backlog#1567 requires to keep failing closed.
Routing between the two decoders cannot key on metadata: RustFS's own writer fills MinIO's slots while storing a RustFS envelope in them, so neither the slot nor the header name distinguishes writers. It keys on the data key's own shape instead, recognizing the two strict RustFS JSON shapes positively and leaving only the remainder to MinIO — so neither decoder is ever handed the other's format. Three round-trip tests caught an earlier slot-based attempt doing exactly that.
Fail-closed is preserved throughout: a scheme that cannot be established still returns None, and the read plan independently classifies the object as encrypted from its markers and refuses to serve it without material, so no path degrades into returning ciphertext as plaintext.
The interop harness also gets a provider reset. The DEK provider is cached process-wide, so a case that ran earlier kept serving its master key to every later case — which silently made the wrong-key negative test unable to fail. It fails correctly now, and the whole suite is meaningful for the first time.
Refs rustfs/backlog#1638.